This invention relates to electro-mechanical connectors for connecting an integrated circuit package to an electronic system in a replaceable fashion; and more particularly, it relates to electrical conductors for use in the above electro-mechanical connectors.
One exemplary electro-mechanical connector of the type to which this invention relates is illustrated in FIG. 1. This particular connector is indicated by reference numeral 10; and in general, it includes a frame 20, a lid 40, and pivotal members 50-1 and 50-2.
Frame 20 is rectangular and has sides 21, 22, 23, and 24. These sides have L-shaped cross sections which define two concentric openings 25 and 26. Opening 25 aligns with the outer perimeter of an integrated circuit package 30; and opening 26 aligns with the outer perimeter of the leads 31 that extend from that package.
Included in each of the sides 21 through 24 are a plurality of electrical conductors 27. The total number of these conductors can vary from connector to connector; and one exemplary total number is sixty-eight (68). One portion 27a of each of the conductors 27 protrudes from the bottom of frame 20 to provide a means for making electrical contact with the electronic system (not shown). Another portion 27b of each of the conductors 27 protrudes through the top of frame 20 and makes electrical contact with corresponding conductors 31 in the integrated circuit package 30. It is the shape of conductor portion 27b to which this invention particularly relates.
Leads 31 in the integrated circuit package 30 connect to an integrated circuit chip 32. Those leads 31 are held in place by a material 33 such as plastic or ceramic. Portions 31a of the leads 31 extend from this material 33; and when package 30 is inserted into opening 25, those portions 31a of the leads 31 align and make electrical contact with corresponding conductor portions 27b in frame 20.
Lid 40, which is also included in connector 10, has hour sides 41, 42, 43, and 44. Those sides have an inner perimeter which coincides with opening 25 of frame 20, and have an outer perimeter which coincides with opening 26 of frame 30. Thus, lid 40 is shaped to lie on top of the lead portions 31a and press them against the corresponding conductor portions 27b in frame 20.
Pivotal members 50-1 and 50-2 provide a means for pressing lead portions 31a and conductor portions 27b together. Each pivotal member has a C-shaped latch portion 51 which consists of a central bar 51a and a pair of ends 51b. Bar 51a fits in fulcrums 45 on top of lid 40; and the ends 51b fit into respective tracks 28 on frame sides 22 and 24.
Each of the pivotal members 50-1 and 50-2 also include a lever portion 52. In operation, after bar 51a is placed in fulcrums 45, a force is manually applied to the lever portions 52. This force causes bar 51 to pivot in the fulcrums 45, causes the ends 51b of the two pivotal members to move in their respective tracks 28, and consequently causes the components 20, 30, and 40 to be pressed together and locked in place.
Consider now in greater detail the shape of the electrical conductors 27 as they are typically made in the prior art. That shape of one of the conductors 27 is illustrated in FIG. 2. As there illustrated, conductor 27 includes a curved member 61 and three straight legs 62, 63, and 64. Member 61 forms an arc of about 160.degree.; and it has two ends 61a and 61b. End 61a is joined to one of the ends of leg 62, and end 61b is joined to one of the ends of leg 63.
Leg 63 lies on its side on the top surface of frame 20; while the curved member 61 along with leg 62 lie in a plane that is perpendicular to the top surface of frame 20. Leg 64 also lies perpendicular to the top and bottom surfaces of frame 20; and it protrudes through those surfaces to join leg 63 at an angle of approximately 90.degree.. Leg 64 also extends beyond the bottom surface of frame 20 to provide a means for making electrical contact with an electronic system (which is not shown).
When the integrated circuit package 30 is locked within the electro-mechanical connector as described above, one of the leads 31 from integrated circuit package 30 pushes against the open end of leg 62. The force that lead 31 exerts against the open end of leg 62 is herein notated as force F; and this force is in a direction perpendicular to and toward the top surface of frame 20. In response to this force F, the open end of leg 62 deflects by a distance y toward the top surface of frame 20.
The distance y by which the open end of the leg 62 deflects may be expressed mathematically as equation A in FIG. 2. In equation A, length L.sub.a is the length of leg 62 as projected into the horizontal plane; length L.sub.b is the length of leg 63; R is the radius of the curved member 61; E is the modulus elasticity of the material from which components 61-64 are made; and I is the moment of inertia of the cross-sectional area of those components 61-64.
As leg 62 is deflected in response to force F in the manner described above, various stresses are generated in legs 62 and 63 and curved member 61. And the largest of these stresses occurs on the surfaces of member 61 at a distance L.sub.T away from the point of member 62 where the force F is applied. This maximum stress is herein notated as S and it can be expressed mathematically as equation B in FIG. 2. In this equation, t is the thickness of conductor 27, L.sub.T is L.sub.a plus R, and all of the other terms are as defined above.
FIG. 3 is a plot of the above equation A for deflection y vs. force F. In that figure, a curve 71 gives the deflection y for conductor 27 under the condition where leg 62 is relatively short; and a curve 72 gives the deflection y in conductor 27 under the condition where leg 62 is relatively long. Also in FIG. 3, the maximum stress S that occurs in the relatively short and relatively long conductors 27 in indicated for the condition where the force F is 200 grams.
More specifically, curve 71 corresponds to conductor 27 having a length L.sub.a of 0.100 inches a length L.sub.b of 0.080 inches, a radius R of 0.020 inches, a thickness t of 0.0142 inches, a width w of 0.025 inches, and a modulus of elasticity of 18.5.times.10.sup.6 psi which is the modulus of elasticity for the copper alloy 725. Curve 72, by comparison, corresponds to conductor 27 having those same physical dimensions with the exception that length L.sub.a equals 0.240 inches and length L.sub.b equals 0.170 inches.
To help visualize the deflection and stress depicted by curve 71, the drawing of conductor 27 in FIG. 2 is scaled to those dimensions which correspond to curve 71. Similarly, the drawing of conductor 27 in FIG. 4 is scaled to those dimensions which correspond to curve 72. Also in FIGS. 2 and 4, a dashed line 27' indicates the manner in which conductor 27 deflects.
However, the distance by which conductor 27 actually deflects in connector 10 cannot be specified by a single number. Instead, that distance can only be specified to be within a certain desired deflection range. For example, in FIG. 3, the desired deflection range for conductor 27 is specified by reference numeral 73 as being from 0.025 inches to 0.045 inches.
In other words, after all of the components 20, 30, and 40 are locked in place, the deflection y of conductor 27 will be somewhere between 0.025 inches and 0.045 inches. This range occurs due to various manufacturing tolerances in the dimensions of the parts of connector 10; and it is also due to manufacturing tolerances in the thickness of the integrated circuit package leads 31.
Suppose now that conductor 27 is deflected by the minimal distance in range 73. When that occurs, one constraint which must be met is that the force F needs to be larger than some minimal force which will insure a reliable electrical connection between conductor 27 and the corresponding lead 31a. In FIG. 3, this minimal force is indicated by reference numeral 74 as being 100 grams.
Conversely, suppose that conductor 27 is deflected by the maximum distance in range 73. When that occurs, one other constraint which must be met is that maximum stress in conductor 27 cannot exceed the yield point stress. The yield point stress is the minimum stress which causes conductor 27 to permanently deform.
Also, another constraint which must be met is that the maximum force F times the total number of conductors in package 10 must not be larger than the force that an average person can reasonably be expected to exert with his fingers. Otherwise, all of the components 20, 30, and 40 could not be pressed together and locked in place. This maximum force per conductor is indicated in FIg. 3 as being 200 grams; and that force times sixty-eight (68) conductors is approximately 25 pounds.
Inspection of curves 71 and 72 shows that neither of the corresponding conductors 27 meets the above constraints. In particular, the conductor 27 corresponding to curve 71 is deficient in that the maximum allowable force F of 200 grams will be exceeded when the conductor is deflected by even less than the minimal distance of 0.025 inches. In other words, the conductor 27 that corresponds to curve 71 is "too stiff".
This "stiffness" is decreased by increasing the length L.sub.a of leg 62 as indicated by curve 72. For that curve, the force F corresponding to the maximum deflection distance of 0.045 inches is approximately 170 grams. However, increasing the length L.sub.a of leg 62 also has several undesirable effects.
One of those undesirable effects is that the maximum bending stress S in conductor 27 increases. For example, the maximum bending stress for the conductor 27 that corresponds to curve 72 is calculated from the above recited equation B for S as being equal to 137,210 psi. By comparison, the maximum bending stress S for the conductor 27 corresponding to curve 71 is only 62,844 psi. Thus, it is apparent that increasing length L.sub.a of leg 62 can result in the yield point stress being exceeded.
Also, another problem that results from increasing the length L.sub.a of leg 62 is that the bottom surface area of the connector frame 20 increases. This is evident from FIGS. 2 and 3. In FIG. 2, the distance L.sub.T is only 0.12 inches; while in FIG. 4 the distance L.sub.T is 0.26 inches. This penalty for increasing L.sub.T is paid four times, once on each side of the connector. Consequently, the number of connectors 10 that can be mounted side by side in a given area is greatly decreased.